1. Field of the Invention
The present invention relates to a semiconductor heterojunction structure which can be used in a semiconductor device for a light emitting element which emits light from blue light to ultraviolet light.
2. Description of the Related Art
Diamond is a thermally and chemically stable material and has a wide forbidden band (5.5 eV) and a large electron mobility (2000 cm.sup.2 /V.multidot.sec). The diamond is widely used as an insulating material, and it will find a variety of applications when it is used as a semiconductive material.
The semiconductor diamond is strongly expected to be used in a device which can be used under a severe environment, a high speed device, a high power device or a blue light emitting element.
In order to utilize the diamond as a semiconductor element, a p- or n-type semiconductor having a low resistance should be obtained and a pn junction should be easily formed.
The semiconductor diamond is in the form of a natural bulk, a high-pressure synthesis bulk or a vapor phase deposition film.
The p-type diamond having a relatively low resistance can be prepared by doping boron (B) in the diamond.
The n-type diamond can be prepared by doping phosphorus (P) or lithium (Li) in the diamond, but it has a high resistance. The n-type diamond having a low resistance has not been prepared yet.
The diamond film prepared by the vapor phase deposition method is extremely expected so as to give a semiconductor diamond device. The vapor phase deposition method comprises exciting a feed gas, causing a vapor phase reaction around a heated substrate and depositing a reaction product on the substrate to form a thin film. A means for excitation includes heat, plasma, light and the like. The excitation by the plasma is classified as an RF grow discharge, or a microwave or direct current discharge according to a means which excites the plasma.
A single crystal diamond film having good quality can grow on a single crystal diamond substrate, particularly by a microwave plasma CVD method.
Currently, a Schottky diode is prepared by utilizing a junction between a metal such as W or Al and the p-type diamond prepared by the vapor phase deposition. FIG. 1 is a sectional view of a Schottky diode having a p-type diamond layer. In FIG. 1, a p-type diamond layer 102 is provided on an insulative diamond substrate 101. A tungsten electrode 103 and titanium electrodes 104 are provided on the p-type diamond layer 102.
A pn junction consisting of the diamond has not been prepared yet.
A semiconductor light emitting element such as a light emitting diode (LED) or a semiconductor laser (LD) has good properties such as high reliability, a high speed and a small size.
The LEDs emitting the light from infrared light to visible light have been developed. The infrared light LED is an essential device in the fields of an optical communication and an optical information processing. The visible light LED is an essential device for various display elements.
Among the visible light LEDs, the red light LED is prepared from GaP (Zn, O), AlGaAs or GaAsP, the yellow light LED is prepared from GaAsP (N) and the green light LED is prepared from GaP (N).
The most important subject of the semiconductor light emitting device is to develop a blue light LED.
The visible light LED which emits the light having wavelength longer than green light has been developed, but the blue light LED has not been developed yet. Then, the color display which has picture elements consisting of LEDs has not been realized.
If the blue light LED having high luminance is developed, a display having a high speed, good control and high reliability or a very thin display can be assembled.
A material having a wide forbidden band (Eg&gt;2.6 eV) is necessary to realize the blue light LED. It is desired that a direct transition and p- or n-type semiconductor having low resistance is prepared from such material.
Currently, a crystal of GaN, 6H-SiC, ZnS, ZnSe or the like is being researched for the blue light emitting element and LEDs are manufactured by way of trial.
However, each of GaN, ZnS and ZnSe does not give the p-type semiconductor having low resistance. Then, good pn junction cannot be formed.
6H-SiC is a most promising material. But it is of a indirect transistion type and has low conversion efficiency. Each known material has advantages and disadvantages, and the blue light LED having the high luminance has not been realized.
It is reported that the luminance of 12 mcd was achieved with a light emitting diode prepared by using 6H-SiC. This value is the highest value in a laboratory level.